Below is info on the version 1.0 circuit bend, though some things apply to version 2.0 as well:
The PS-3 uses a big mystery IC as it's main processor and sound generator. It can do 8-note polyphony, sending out two octaves separately.
Yamaha used these two octaves, either mixed or separated, and runs them through several capacitor filters to create the various sounds. For two sounds (Brass and Guitar) the VCF is used to get that 'wah' tone. The VCA is only used for the Vibraphone sound to create a subtle tremolo.

At first I felt like completely changing the signal path, making my own custom circuits and though that would've made something great for sure I also really wanted to keep this thing as original as possible while expanding it's functionality. So I opted to route various signals through a connector to an external "breakout box".

I am definitely planning on getting a second PS-3, gutting that one entirely and giving it a typical analog synthesizer signal path with all the controls available.

Here's a block diagram of the PS-3 for reference:

The diagram also shows the pin numbers and involved component numbers, which was extremely useful in finding modding points.
I know there's a service manual floating around -somewhere-, but unfortunately I haven't been able to get my hands on one. This also meant I didn't have a schematic, so I made my own of certain parts that I modded.

Filter:
First things first: keep in mind that the PS3 uses a wierd minus 9 volts power supply, essentially making "ground" a positive 9V and making everything power supply related reversed. This is really fucky the first time so pay attention!
Here's the rough schematic of the VCF and the mods I made:

I replaced the resonance feedback resistor with a variable one, 10k gives you none to tunable self-oscillation that eventually gets so loud it distorts the audio pretty neatly.
The cutoff frequency CV is normally provided by the envelope, but I put it through a 10k mix knob ("depth") with a second knob in parallel wich acts as a variable voltage. Both voltages are passively mixed together.

Envelope:
The PS-3's envelope is a rather simple capacitor-based one so I left that one alone. . At first I was planning to customize it to allow for setting the attack and decay rates, but I settled for a single envelope-depth knob that mixes in the envelope voltage. I also still need to add external CV to the breakout box, so I can do more fancy stuff with the VCF.

Waveforms:
To get some sort of variation in the waveforms I opted for 2 variable resistors so I can passively mix the two octaves of square wave output by the sound chip.
Below is a crude schematic of this section of circuitry:

On pins 17 and 18 you can find the audio outputs of the big controller/sound chip, these two outputs go to two opamp buffers, one outputting just one square wave, the other passively mixing the two square waves to create a crude sawtooth. The resistors in this passive mixer are the ones you want to replace, named 'Ra' and 'Rb' in the schematic.
I also added a third, external, audio source so I can run stuff through the filter which is loads of fun! :D